Materials that are electrical conductors and are also relatively transparent (or highly transmissive of light) are extensively used in electronics applications and in a wide variety of electronic devices. Such materials may be labeled transparent conducting (TC) materials and are often provided in a multi-layered device or substrate using thin film technologies. For example, one of the most widely used commercial TC materials is indium-tin oxide (ITO), which may be applied as a thin film or layer (e.g., a layer of material that has a thickness less than about 1 micron) to a supporting substrate of glass, ceramic, plastic, or other light transmitting material. TC materials are used in solar or photovoltaic cells and in many other electronic devices such as in touch screens and the like. TC materials are also widely used in organic electronics applications including organic light emitting diodes (OLEDs) in which the emissive electroluminescent layer is composed of a film of organic compounds.
To function properly, the TC materials or substrates containing thin films of such materials typically have desired levels of transparency and electric conductivity. Additionally, it is often desirable that the TC substrates be flexible or otherwise resist cracking. Further, many applications such as OLED devices should be protected from moisture and gases as the functionality of a TC material layer may be ruined or degraded by water or exposure to oxygen and other gases. For example, a photovoltaic cell may be exposed to flexing and bending due to wind and other forces and may also be exposed to moisture and other environmental elements that can degrade effectiveness of the cell. Hence, there have been significant efforts to provide thin film devices or substrates with TC material layers that meet transparency and conductivity while including separate permeation barriers and satisfying other demands such as lower costs and ease of manufacture.
Providing a TC material with high conductivity and transparency has proven challenging to thin film device developers. In general, conductivity and transparency are inversely related in most common forms of transparent conductors such as transparent conducting oxides or TCOs, and, as a result, a material with higher conductivity may have lower transparency and vice versa. ITO has been widely used as the transparent conductor in electronic devices because it provides a balance between transparency and conductivity. However, indium is relatively expensive as it is typically produced as a byproduct of zinc mining and it is a rare metal. Less expensive alternatives such as doped zinc oxide (ZnO) and other TC materials have been used in some devices as the transparent conductor but process sensitivities and lower conductivity/transparency performance issues have limited the commercial acceptance of these substitutes for ITO. Attempts to lower the costs associated with use of ITO and other TC materials have included using less material, i.e., depositing a thinner TC layer. However, the use of thinner layers of the TC material when combined with repeated bending and other stresses often results in cracks in the brittle TC material, and these cracks often unacceptably increase resistivity (i.e., lower conductivity) of the TC layer.
In some efforts to overcome existing limitations of the TC materials, thin film device developers have used films of a metal such as silver, gold, or copper in conjunction with the TCO. The film of metal is provided to enhance the conductivity of the TCO layer while attempting to maintain a desired level of transparency. In such devices, the substrate can be designed to have a metal layer and a TCO layer that provides a higher light transmission than the metal layer or film by itself and also higher conductivity than the TCO material by itself. As a result, these devices may use thinner TC layers (i.e., use less TCO material) while providing the same or enhanced conductivity and/or light transmission. These devices have not been widely adopted commercially in part because these devices often have to be relatively thick. Specifically, the deposition of metals such as silver may involve islanding problems that make it difficult to apply a thin film of the highly conductive metal. Additionally, a non-conformal coating of such metal has decreased conductivity. Hence, these devices typically have a metal layer applied over the transparent conductor that is 7 to 10 nanometers (nm) or more thick to achieve desired conductivity.
In addition to providing transparency and conductivity, many electronic devices such as photovoltaic devices and devices with OLEDs must be protected from environmental contaminants such as moisture and air. A substantial cost of many thin film electronic devices is associated with incorporating a barrier or coating to try to prevent oxygen, water, and/or other environmental contaminants from reaching sensitive electronic components such as the TC layer. While relatively thick metal coatings are used to provide such protection in some packaging, these layers are not transparent which makes them unacceptable for many applications such as solar cells, and the permeation barrier may still not be adequate for the particular device. Particularly, some applications (e.g., organic-based electronic devices such as OLEDs for displays or organic photovoltaics) may use permeation rates for water/oxygen less than about 10−6 grams/m2-day. As a point of comparison, typical permeation rates of water through 4-mil PET is about 10 grams/m2-day at standard test conditions of 40° C. and 40 percent relative humidity. Layers of inorganic coatings such as oxides and nitrides (e.g., aluminum oxide, silicon oxide, and silicon nitride) are often used in conjunction with a plastic substrate to reduce the permeation by a factor of 10 to 1000. Permeation drops as the inorganic thickness increases, but it reaches a lower limit for film thicknesses of a few hundred to about 2 thousand Angstroms.
The lower limit for permeation is associated with the inevitable presence of film defects such as pinholes, cracks, scratches, and the like. This has been found to be true for a wide range of materials and deposition techniques such as physical vapor deposition by evaporation or sputtering and for plasma enhanced chemical vapor deposition. In an attempt to disconnect the pinholes and cracks, layers of organic materials have been incorporated in some devices, but research indicates that due to the relatively thick geometry of these layers the water/oxygen transport was simply delayed and not blocked (i.e., the overall permeation rate is not substantially reduced). Hence, the diffusion rate through these multiple layer configurations is approximately the same as through a single inorganic layer of sufficient thickness. Another permeation barrier technique has attempted to address the problem with pinholes by constructing a conformal pinhole-free inorganic oxide coating using atomic layer deposition (ALD) to apply the barrier layer, and, in tests, these barriers achieve significant improvements in the permeation rate of water/oxygen. Unfortunately, the use of oxides and similar materials is not acceptable in many settings where high conductivity is desired as these materials have high resistivity. Additionally, oxide layers are often brittle and may often crack when stresses are applied or may propagate cracks that form in the underlying layers.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings.